linux-kernel - Re: [PATCH v2 4/5] ACPI/PCI: Warn if we have to "guess" host bridge node information

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Date:	Mon, 28 Apr 2014 15:24:39 -0600
From:	Myron Stowe <myron.stowe@...il.com>
To:	Borislav Petkov <bp@...e.de>
Cc:	Myron Stowe <myron.stowe@...hat.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	linux-pci <linux-pci@...r.kernel.org>,
	Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
	Aravind Gopalakrishnan <aravind.gopalakrishnan@....com>,
	kim.naru@....com, daniel@...ascale.com,
	Thomas Gleixner <tglx@...utronix.de>, mingo@...hat.com,
	hpa@...or.com, x86 <x86@...nel.org>, sp@...ascale.com,
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 4/5] ACPI/PCI: Warn if we have to "guess" host bridge
 node information

On Sun, Apr 20, 2014 at 4:21 AM, Borislav Petkov <bp@...e.de> wrote:
> On Fri, Apr 18, 2014 at 08:53:39PM -0600, Myron Stowe wrote:
>> The vast majority of platforms are not supplying ACPI _PXM (proximity)
>> information corresponding to host bridge (PNP0A03/PNP0A08) devices
>> resulting in sysfs "numa_node" values of -1 (NUMA_NO_NODE) [1]:
>>   # for i in /sys/devices/pci0000\:00/*/numa_node; do cat $i; done | uniq
>>   -1
>>
>>   # find /sys/ -name "numa_node" | while read fname; do cat $fname; \
>>     done | uniq
>>   -1
>>
>> AMD based platforms provide a fall-back for this situation via amd_bus.c.
>> These platforms snoop out the information by directly reading specific
>> registers from the Northbridge and caching them via 'alloc_pci_root_info'.
>>
>> Later during boot processing when host bridges are discovered -
>> 'pci_acpi_scan_root' - the kernel looks for their corresponding ACPI _PXM
>> method - drivers/acpi/numa.c::acpi_get_node().  If the BIOS supplied a
>> _PXM method then that node (proximity) value is associated.  If the BIOS
>> did not supply a _PXM method *and* the platform is AMD based, the
>> fall-back cached values obtained directly from the Northbridge are used;
>> otherwise, "NUMA_NO_NODE" is associated.
>>
>> There are a number of issues with this fall-back mechanism the most
>> notable being that amd_bus.c extracts a 3-bit number from a CPU register
>> and uses it as the node number.  The node numbers used by Linux are
>> logical and there's no reason they need to be identical to settings in the
>> CPU registers.  So if we have some node information obtained in the normal
>> way (from _PXM, SLIT, SRAT, etc.) and some from amd_bus.c, there's no
>> reason to believe they will be compatible.
>>
>> This patch warns when this situation occurs:
>>   pci_root PNP0A08:00: [Firmware Bug]: No _PXM; guessing node number 0
>>
>> [1] https://bugzilla.kernel.org/show_bug.cgi?id=72051
>>
>> Signed-off-by: Myron Stowe <myron.stowe@...hat.com>
>> ---
>>
>>  arch/x86/pci/acpi.c |    6 +++++-
>>  1 files changed, 5 insertions(+), 1 deletions(-)
>>
>> diff --git a/arch/x86/pci/acpi.c b/arch/x86/pci/acpi.c
>> index 01edac6..80c09ba 100644
>> --- a/arch/x86/pci/acpi.c
>> +++ b/arch/x86/pci/acpi.c
>> @@ -489,8 +489,12 @@ struct pci_bus *pci_acpi_scan_root(struct acpi_pci_root *root)
>>       }
>>
>>       node = acpi_get_node(device->handle);
>> -     if (node == NUMA_NO_NODE)
>> +     if (node == NUMA_NO_NODE) {
>>               node = x86_pci_root_bus_node(busnum);
>> +             if (node != NUMA_NO_NODE)
>> +                     dev_info(&device->dev, FW_BUG "No _PXM; guessing node number %x\n",
>
> Hmm, I'm not really convinced this message is user-friendly enough. Can
> we be more descriptive here please?
>

How about -
  dev_info(&device->dev, FW_BUG "no _PXM; falling back to node %d from
hardware (may be inconsistent with ACPI node numbers)\n", node);

> --
> Regards/Gruss,
>     Boris.
>
> Sent from a fat crate under my desk. Formatting is fine.
> --
> --
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